The present invention relates in general to power generation devices, and, more in particular, to a wind-driven power generation device.
Windmills have been known since ancient times. These devices extract power from the wind. Usually, the power is used in driving pumps for irrigation or supplying electrical power in rural areas.
Some large scale wind turbines have been successful. One of these was the Smith-Putnam wind turbine generator built in Vermont in the early 1940's. This system had a blade span of 175 feet and produced 1.25 megawatts of electricity in a 32 mile per hour wind. The unit was abandoned upon a blade failure in favor of conventional electrical generating plants that were more cost effective at that time. With the increase in energy costs, the attractiveness of wind power has improved.
Recently consideration has been given to using wind power generators to supply electrical energy for sophisticated requirements. A host of problems, however, attend such an attempt. Wind is incredibly variable. Wind varies from geographical location to geographical location and from season to season. Some areas are blessed with a considerable amount of wind. Others are wind poor. Wind velocities and direction fluctuate broadly in short periods of time. In areas where considerable wind exists, the diurnal changes in wind velocity can vary from almost nothing to a considerable value. A mean wind speed is attendant with frequent gusts and lulls. The wind velocity varies considerably in elevation close to the ground.
The lack of constant wind from a constant direction makes power generation for electrical utility purposes seem difficult. Electrical power for utilities must be of extremely high quality. By way of example, a utility generated power must be held extremely close to 60 cycles per second. If it is not, the power is totally unsatisfactory. This means that in a wind generating system some means must exist to assure constant generator speed. Synchronous generators can obtain this end, but they must be powered by a system that supplies considerable power for all wind conditions if overall efficiencies are to be optimized. The generators cannot be permitted to devote energy either to slowing down the drive or speeding up the drive. The generators motoring the drive system can also place unusual stresses on the roots of the propeller blades.
A considerable amount of power exists in the wind at high wind speeds. The traditional approach to wind power generators, however, has been to shut down the generators above certain speeds. Shutdown was thought necessary to avoid destruction of the wind power generator by the extremely high forces produced by high winds.
The traditional approach in wind power generators operating in parallel with a utility system has been to vary blade pitch to achieve good efficiencies. These generators operate at constant propeller speed, regardless of wind speed, up to some predetermined limit of wind speed, whereupon shutdown occurs. This maintenance of propeller speed constant does not optimize the power generated. Optimization of power requires matching of the speed of the propeller with a corresponding wind velocity. Different wind speeds require different propeller speeds for optimum power.
In wind turbine generators, it is extremely important that the propeller pitch change rapidly in response to gusts of wind. One approach that has been taken in the past senses the overloads produced by gusts and generates a responsive signal. This signal operates a pitch control mechanism that feathers the propellers. The lag time in this system is substantial, some 5 seconds. To avoid this lag time, it is highly desirable that the propellers change pitch directly as a result of an overload sensed by the propellers.
Overload conditions in the past have been relieved by brakes on the generator of the wind turbine generators. The overload is sensed and the brake applied. Again the lag time is too high to assure satisfactory response to overload conditions.
Another problem attendant with previous designs is in their use of gear boxes and mechanical drive components, such as shafts and belts to transmit power from propellers to generators. Because of the speed range over which these devices must operate for effective power generation, they are susceptible to resonance. Furthermore, the drive trains of such systems become cumbersome and expensive. It is also important to avoid having too much mass above ground in order to increase tower resonant frequency and to reduce the strength and rigidity requirement of the tower.
All wind power generators have more than one propeller blade. The common approach in the past has been to feather all the blades as a unit. If one of the blades sticks when feathering is mandated, the complementary blades will not feather and the generating system could destroy itself.
It is important, also, to have a tower design that is rigid enough to reduce the amplitude of resonant vibrations of the tower, and to have a design that avoids unnecessary cyclic loads on the propeller blades. Vibration pulses occur each time a propeller blade passes the tower. For proper operation without excessive power fluctuations and large resonant exciting forces due to wind shadow on the propeller, the propeller should operate upwind of its tower. By placing the propeller upwind, tower shadowing of the blades does not occur and the resulting pulsing on the blades is eliminated. Nonetheless, even with an upwind facing propeller, the tower is cyclically excited.
Another problem with these systems occurs because of the tremendous size required for an efficient generating system. This results in extremely difficult blade design. Fatigue obviously is an important factor especially in view of the fact that the blades operate over a wide speed range and a wide loading range, both of which vary rapidly with time. Propeller systems with two blades have proven to be unsatisfactory because the substantial load required on each propeller blade to produce reasonable output results in high cyclic loads applied to the propeller blades and the tower. The fatigue problem associated with the variable speed and loading has not been adequately solved by aluminum or steel blades. Combinations of aluminum and fiberglass will not prove satisfactory either.